1. Field of the Invention
This invention relates to the manufacture of integrated-circuit (IC) parts. More particularly, this invention relates to the testing of such parts at the end of the manufacturing process.
2. Description of the Prior Art
Integrated-circuit chips conventionally are packaged as completed products known as "parts". These parts generally comprise a large number of external pins through which electrical connection may be made to the circuitry on the chip. At the end of the manufacturing process, the pins of the completed parts typically are connected to automatic test equipment and given a series of electrical tests to determine the performance characteristics of the integrated circuitry. Frequently, such tests are used to "grade" the parts into commercial categories of performance capabilities, commonly identified by letters such as J, K, L, etc. U.S. Pat. No. 4,646,238 (Carlson et al) discloses how such grading fits into the computer-controlled planning of production and testing of IC parts.
Integrated circuits generally are formed on thin wafers of silicon in a series of alternating photographic and deposition processes which create hundreds and possibly thousands of identical circuits on each wafer. The wafers are processed in "lots" which typically will include 21 wafers in a so-called "boat". If, for example, each wafer included 600 IC chips or "dice", there would be 12,600 such chips in a wafer lot.
After circuit formation, the wafers are probed and each circuit powered up in order to make preliminary tests to determine which dice are non-functional and must be discarded. Frequently at this wafer stage of the process, certain elements of the integrated circuitry may be trimmed, as by laser trimming of resistors, to bring the values of such elements to within specified limits.
Thereafter, the individual dice of each wafer are separated, inspected for visual defects, and mounted in respective packages as described above, i.e. packages having a large number of external pins for making electrical connections to the integrated circuitry when the part is assembled together with other components to make a complete device. Very fine wires within each package connect these pins to bonding pads on the IC chip, which in turn make connection to corresponding points of the integrated circuitry.
After packaging, the parts are tested and graded according to their electrical performance against applicable specifications. The simplest grades may be tested only at room temperature. Higher quality grades, such as those for military and aerospace applications, generally must pass tight performance standards while being subjected to extreme environmental conditions, such as very high and very low temperatures. In addition, the critical characteristics of higher grade products often must be tested in such a way to assure that a parametric drift or temperature coefficient is not out of specifications with respect to an entire temperature range. Drift tests generally include at least three separate tests for each part, at different temperatures; sometimes as many as seven or eight separate tests may be required for each part.
When the parts are being drift-tested, it is essential to be able to establish the identity of each part in such a way as to positively associate each part with a corresponding set of test data covering different temperature conditions. Thus, as the parts are shifted from one test condition to another during a drift test, the identity of each part must in some fashion be continuously monitored or otherwise determined positively, so that errors will not be made in assigning the sequential sets of test data to particular parts.
Conventional IC test equipment frequently includes apparatus known as a "handler" into which parts to be tested are loaded for carrying out tests automatically. Prior to insertion into a handler the parts generally are carried in plastic tubes (sometimes called "sticks") which are elongate devices holding about 25 to 100 parts each, all arranged in a row. The parts are stripped from the tubes and loaded into the handler where they are physically transported through the test chamber while being brought up to the specified temperature. Each part thereafter is connected, in sequence, for a very short time to the test equipment circuitry which at high speed typically measures voltages and currents under a computer program control. The handlers include facilities to reassemble the parts back into tubes after a test run has been completed. Subsequent tests at different temperatures are carried out in the same fashion. A "test lot" for a handler typically may total 1500 parts.
The results of the computer-controlled electrical tests performed on the parts are stored in memory, and the procedures used are carried out so as to provide that the test data can in some fashion be associated with the respective parts. As noted above, for drift tests a series of readings must be made at different conditions. It is necessary that each set of such sequential readings stored in memory for a particular part always be positively associated in the memory with that same part, for otherwise the final analyses of the test data would yield faulty results.
Various techniques have been developed for establishing and/or monitoring the identity of the individual parts being tested, so as to assure that the test results for any one part are correctly associated with that part. Although such prior techniques have in some respects been effective, the overall results have not been fully satisfactory for many applications. For example, some prior techniques require that the carrying tubes have to be kept in their original sequence, and similarly that the parts be kept in their original sequence in each tube. As will be evident from the description hereafter, the present invention avoids the need for such ordering of the parts and/or tubes.
Accordingly, it is a purpose of this invention to provide improved techniques for marking and subsequently identifying IC parts as they pass through the test procedures at the end of an IC manufacturing process.